Many hydrocarbon wells are produced by electrical submersible well pump assemblies (ESP). A typical ESP includes a centrifugal pump having a large number of stages, each stage having an impeller and a diffuser. An electrical motor couples to the pump for rotating the impellers. A pressure equalizer or seal section connects to the motor to reduce a pressure differential between lubricant in the motor and the hydrostatic pressure of the well fluid. The pressure equalizer has a motor lubricant passage leading from a flexible barrier such as a bag or bellows into the interior of the motor. The motor lubricant passage is always open to communicate well fluid pressure applied in the pressure equalizer to the flexible barrier to the motor lubricant in the motor.
With most prior art ESP's, the pressure equalizer or seal section is located between the motor and the pump. In others, the pressure equalizer is mounted below the motor. During a prior art installation using a below motor pressure equalizer, the pressure equalizer may be initially filled with lubricant and suspended vertically from a rig at the well site. The motor is then lowered onto the equalizer and secured. Then motor lubricant may be pumped in from the lower end of the motor and upward through the motor. Alternately, the motor may be evacuated by a vacuum pump, then filled from the top.
The weight of the motor lubricant filled into the motor while the assembly is suspended above the well would act hydrostatically on the bellows of the pre-filled pressure equalizer, possibly causing the bellows to become fully extended. If fully extended before lowering into the well, and if the motor is completely full of lubricant, the bellows would not be able to further extend due to an increase in temperature, requiring some of the lubricant to be expelled through a check valve. The combined equalizer and motor would thus be over-filled with lubricant before the assembly is lowered into the well. The preferred position of the bellows prior to lowering the assembly into a well provides adequate expansion capacity of the bellows in cases of low pressure and high temperature while also maintaining adequate contraction capacity in cases of high pressure and low temperature.
Also, if multiple motors are in tandem, the assembly can be quite lengthy, more than 100 feet. The total length, including the pressure equalizer, could be greater than the distance from the blocks of the rig to the wellhead. If the lower end of the assembly is lowered into the wellhead in order to accommodate the length of the assembly during motor lubricant filling, the procedure becomes difficult if lubricant is pumped from the lower end, which would require access to the lower end of the assembly.